Recent development of nano-scale technology enables the fabrication of molecular bridges sandwiched between two metallic electrodes. Although the conductance calculation has been performed by many researchers, internal current distribution, which reveals remarkable quantum features with a number of possible novel functions, has not been discussed so far.

The authors calculated internal current distributions of molecular bridges with tight-binding models (Figure). The finding is that the source-drain current induces large loop current (Induced Large Loop Current : ILLC) inside the molecular region when a certain condition is satisfied. The magnitude of the induced loop current can be of the order of several tens of that of the source-drain current. The loop current couples with the external magnetic field or the local moment embedded in the molecule, thereby causing novel functions as a quantum device element controlled by the phase of the molecular orbitals.

From numerical calculations, the feature of the loop current is clarified. The ILLC appears when the incident electron energy nearly coincides with the degenerate or nearly degenerate energy levels of the molecule. The magnitude and the direction of the induced loop current change sharply around the degenerate energy level of the molecule. The magnitude of ILLC takes its maximum just above or below of the degenerate energy level, with changing its direction at the level. For nearly-degenerate case, the feature of the ILLC is qualitatively different from the degenerate case: the magnitude of the current has its maximum value when the electron energy is located just between the nearly-degenerate levels of the molecule.

This feature can be understood by considering the scattering wavefunctions of the whole open system at the molecular region. The phase difference between the degenerate (or nearly-degenerate) MOs coupled with electrode scattering states determines the behavior and the appearance of the ILLC.

The direction of the ILLC is controlled by the external magnetic field or the gate bias, and strongly couples with the embedded moment. This feature provides novel functions of the molecular bridges as quantum devices.

Figure: The model and the typical example of the Induced
Large Loop Current (ILLC) inside the molecular region. The current
is illustrated by the arrows. Electron energy is set to be E=-1.73t.
The electrode-molecule coupling is set as the half of the transfer integral
inside the electrodes and the molecule t.